Pulsed neutron generator



March 14, 1967 A. H. YOUMANS ET AL 7 3,309,522

PULSED NEUTRON GENERATOR Original Filed Feb. 18, 1963 PLATE he CORONASUPFLYY REGULATOR 3 Sheets-Sheet 1 35 as 32 2s 39 \6 X [I A A L 52 2457"!= 51 E1 T 23A 23 FIG. 3

ERIC C. HOPKINSON ATTORNEY March 14, 1967 A. H. YOUMANS ET AL 3,309,522

PULSED NEUTRON GENERATOR Original Filed Feb. 18, 1963 5 Sheets-Sheet 2FIG. 4

POWER INPUT INDUCTION CORONA PLATE SUPPLY REGULATOR L 45 6 r27 m 39 a533 32 23A c E} 37 z "-3 LJ t |o2 L MG H L 25 23 1* zaA -l- I00 NEGATIVEPULSER INVENTORS ARTHUR H. YOUMANS ERIC C. HOPKINSON ATTORNEY March 14,1967 A. H. YOUMANS ETAL PULSED NEUTRON GENERATOR Original Filed Feb. 18,1963 3 Sheets-Sheet 5 POWER INDUCTION CORONA INPUT PLATE REGULATORSUPPLY -l- 39 35 33 a2 2 A p3 L r4 3 A r\ n A 77) 5 37 L, v v v v I02 7m 4 @27 c 24 23 (W A 33 F; 25

0 /u4 35) L5 327 4 23A\ mm/ (:EUN-ESRZTOR Gav- 1) ire? F 22 A H 1 1? Q lINDUCTION PLATE SUPPLY |24\ A A PULSE GENERATOR INVENTORS ARTHUR H.YOUMANS ERIG C. HOPKINSON ATTORNEY United States Patent 3,309,522 PULSEDNEUTRGN GENERATOR Arthur H. Youmans and Eric C. Hopkinson, Houston Tex,.assignors to Dresser Industries, Inc., Dallas Tern, a corporation ofDelaware Continuation of application Ser. No. 259,073, Feb. 18, 1963.This application May 13, 1965, Ser. No. 456,902

26 Claims. (Cl. 250-845) This is a continuation of the copendingapplication Ser. No. 259,073, filed Feb. 18, 1963, by Arthur H. Youmansand Eric C. Hopkinson, and now abandoned.

This invention relates to methods and apparatus for causing ion beamaccelerator tubes to generate pulses of neutrons at a very highfrequency, and is particularly directly to methods and apparatus of suchcharacter which are capable of being used in the logging of boreholes inthe earth.

Various methods and apparatus employing sources of radiation for theinvestigation of subsurface earth formations are now well known.Generally, most of the radioactive well logging apparatus now in useincludes either a source of gamma rays, such as an encapsulated quantityof cesium-137, or a source of neutrons such as an encapsulated mixtureof radium and beryllium. Although these capsules of radioactive materialare still in common use for well logging purposes, so-called artificialsources of radiation are becoming increasingly popular because they maybe turned off when not in use, and because they provide relativelymonoenergetic radiation of a particular character. Typical of theseartificial sources is the static atmosphere ion accelerator tubedescribed in US. Patent No. 2,689,918, which issued Sept. 21, 1954, toA. H. Youmans, and which is designed to provide a substantial output ofhigh energy neutrons by means of the now well-known D-T reaction.Accelerator tubes of this type may be powered by a belt-driven,electrostatic generator, such as the well known Van de Graaff highvoltage generator.

Generally, most accelerator tubes used in well logging instrumentscomprise a steel jacket adapted to house a sealed or static atmospherecomposed of one or both of the heavy isotopes of hydrogen at a very lowpressure. This atmosphere may be considered functionally as divided intoan ionization region and an accelerating region. Thus the acceleratortube includes structure for ionizing the atmosphere within the so-calledionization region. These ions are then accelerated at high speeds, byother tube structure, into a target which also contains one or both ofthe heavy hydrogen isotopes, and the resulting reactions produceneutrons. If both the atmosphere and the target contain only deuterium,then neutrons having energies of approximately 2.2 mev. will be producedby virtue of the well known D-D reaction. Alternately, if the target isimpregnated with tritium as described in the aforementioned YoumansPatent No. 2,689,918, the neutrons having energies of approximately 14.4mev. will be produced as a result of the well known D-T reaction.

Several techniques are presently used in logging operations employing anartificial source of neutrons. For example, the formations may bebombarded with neutrons and a measurement may be made of the number andenergy of gamma rays arising from either inelastic scattering, orcapture, of the bombarding neutrons by the nuclei of the formationmaterials. Alternately, high energy neutron bombardment may excite theformation nuclei so as to cause these excited nuclei to emit decay gammaradiation, and a measurement of the energy and decay rate of suchradiation will indicate the identity of the excited nuclei. Then again,measurements of the distribution of the neutron flux are often made withrespect to time or space.

Patented Mar. 14, 195? It may be readily seen that, when the formationssur rounding a borehole are irradiated by neutrons from an accelerator,gamma radiation will be produced in the formations simultaneously as aresult of all of the foregoing nuclear reactions. However, if theaccelerator is caused to emit neutrons in discrete bursts, or pulses,then one or more classes of gamma radiation may be distinguished andidentified. In particular, if the earth formations are irradiated withneutrons during repetitive, relatively short intervals of time wherebysuccessive operating cycles are defined, then each such cycle willconsist of an irradiation interval followed by a quiescent interval.Each such quiescent interval may be comprised of a first period whereinneutrons from the accelerator may be slowed, diffused, and ultimately becaptured by nuclei of the formation substances so irradiated, and asecond period wherein radioactive elements formed by neutroninteractions may exhibit a product of radioactive decay. The averagelifetime of a neutron in a vacuum is about 13 minutes, but in ordinarymaterials the lifetime of neutrons is much shorter. In common earthmaterials, the average lifetime of neutrons ranges between extremes ofmicroseconds (more or less) for salt water, to perhaps 4000 microsecondsor more in quartzite. Thus, in order for the operating cycle of a pulsedaccelerator source to be useful and meaningful, it is essential that theirradiation time interval be very short if the exponential decay of theneutron flux is to be measured, or if the so-called prompt gammaradiation is to be distinguished from decay gamma radiation emitted byactivated nuclei. in partiallar, such irradiation intervals shouldpreferably be of the order of 5 to 50 microseconds in duration, sincefast neutrons l mev. or greater) are generally slowed to thermal energyin 10 to 100 microseconds, depending upon environment. For example,hydrogenous materials will exercise greater slowing effect on fastneutrons than will other substances, and neutrons in and near aliquid-filled borehole will be slowed to thermal energy before thoseneutrons which have penetrated the formations.

The operation of a neutron source composed of an accelerator tube and abelt-driven electrostatic generator is never a simple matter, even whensuch a source is located in an ideal environment. Many system parametersmust be properly selected and maintained in proper bal ance, as willhereinafter be made apparent, before a steady neutron output can besatisfactorily attained, and if the generator is cyclically actuated,these parameters tend to quickly fall into imbalance. Moreover, sinc thestability of this type of neutron source is particularly affected bychanges in environment, and since well logging equipment is subjected togreat changes in environment, attention has heretofore been almostexclusively directed to stabilizing the neutron output, rather thantowards producing a pulsed output by deliberately cycling the source.Such cycling of the source, which hasheretofore been achieved withoutunduly affecting the stability of the system, has been generally limitedto pulses of greater than 0.1 second in duration, and at a pulsefrequency of about 5 per second.

These disadvantages of the prior art are overcome with the presentinvention, and novel methods and apparatus are provided herein whichobtain pulses of neutrons as short as 5 microseconds in duration at apulse frequency at least as great as 10,000 pulses per second; Moreover,with the present invention, system stability is not adversely affected,power is conserved rather than wasted, and both the duration of eachpulse and the magnitude of the quiescent interval are effectivelycontrolled.

The advantages of the present invention are preferably attained, in aneutron source composed of an ion beam accelerator tube and a Van deGraaff electrostatic generator, by maintaining all other systemfunctions at a stable condition of operation while pulsing only the flowof ionizing electrons in the ionization region of the accelerator tube.Particular methods and apparatus for interrupting the ionization current(as it is generally called), without adversely affecting the balance ofthe system, will hereinafter be described in detail.

Accordingly, it is an object of the present invention to provide novelmethods and apparatus for obtaining pulses of neutrons at a very highpreselected frequency and having a very short duration.

It is also an object of the present invention to provide novel methodsand apparatus for pulsing an artificial source of neutrons disposed insubsurface well logging apparatus, at a very high preselected frequencyand having a very short duration.

A specific object of the present invention is to provide novel methodsfor pulsing the ionization supply of an ion beam accelerator tubeconnected to an electrostatic generator, in a manner such that saidaccelerator tube is caused to emit discrete pulses of neutrons at a veryhigh preselected pulse frequency and of a very short duration.

Another specific object of the present invention is to provide novelapparatus for pulsing the ionization supply of an ion beam aceleratortube connected to an electrostatic generator, in a manner such that theaccelerator tube is caused to emit discrete pulses of neutrons at a veryhigh preselected pulse frequency and of a very short duration.

These and other objects of the present invention will be apparent fromthe following detailed description wherein reference is made to thefigures of the accompany ing drawings.

In the drawings:

FIGURE 1 is a schematic diagram of a typical ion beam accelerator tubeand electrostatic generator arranged to produce neutrons.

FlGURE 2 is a simplified representation of the accelerator tube andelectrostatic generator shown in FIGURE 1, and adapted to incorporate aradiation responsive means for triggering the accelerator tube.

FIGURE 3 is a similar representation of an accelerator tube andelectrostatic genertaor, including a trigger tube interconnected fortriggering the accelerator tube.

FEGURE 4 is a schematic representation of a modified form of theapparatus depicted in FIGURE 3.

FIGURE 5 is a schematic representation of another modified form of theapparatus depicted in FIGURE 3.

FIGURE 6 is a schematic representation of the apparatus depicted inFIGURE 1, including triggering apparatus interconnected with thesuppressor ring system in the accelerator tube.

FIGURE 7 is a schematic representation of a modified form of theapparatus depicted in FIGURE 6.

FIGURE 8 is a schematic representation of another modified form of theapparatus depicted in FIGURE 3.

In those forms of the present invention chosen for purposes ofillustration in the drawings, FIG. 1 shows an artificial neutron sourcecomposed of an accelerator tube 2, such as the static atmosphere ionaccelerator depicted in the aforementioned Youmans Patent No. 2,689,-918, and a belt-driven electrostatic generator 4, such as a Van deGraaff high voltage generator. As depicted, the accelerator tubeincludes a gas-tight jacket 20 which is adapted to house an atmosphere21 composed of either deuterium or tritium (or a mixture of both), at avery low pressure. .Disposed generally at the axial center of theaccelerator tube 2 is an ionization supply composed of an anode 24 and acathode 23. The anode 24 is preferably formed of a single wire orrelatively thin rod-like electrode, and the cathode 23 is preferablyformed of a circular body of fine wire mesh or screen disposedcircumferentially about the wire anode 24. The cathode 23 is preferablymounted on or supported by a metal tube-like column 23A. Included withinthe accelerator tube 2, there may be found a belt-shaped target 22 whichis generally formed of a thin strip of titanium mounted on the innersurface of the jacket 29 in a manner to surround or encircle the cathode23 and anode 24. The target is necessarily impregnated with eitherdeuterium or tritium, or a mixture of both, as will hereinafter beexplained. Between the target 22 and the cathode 23 may also be foundone or more electrodes, which are generally referred to as suppressorrings 12, for suppressing secondary electron emission from the target 22as will also be hereinafter explained.

The electrostatic generator 4 is preferably composed of a cylindricaltank 30, which is electrically connected to the jacket 20 of theaccelerator tube 2 by way of ground or common, and which is preferablyadapted to house a so-called lower pulley 38 and an upper pulley 36.These two pulleys are arranged to support and to drive a continuous belt37 formed of non-conductive material such as leather or cloth, andcontaining or hearing small, regularly sized and spaced segments 37A ofan electrically conductive substance as shown in FIG. 1. The lowerpulley 3 3 is connected, for rotation purposes, to a driving mechanismof drive-shaft 40 of an electric motor 5. The upper pulley 36 vmay besupported by a two-section electrode assembly hereinafter referred to asthe lower hollow electrode 35 and the upper hollow electrode 32. Thesetwo hollow electrodes are electrically separated from each other by aninsulator 33, and from the tank 30 and the jacket 20 by the insulatingsocket 27 which connects the electrostatic generator 4 and theaccelerating tube 2. As

shown in FIG. 1, the lower hollow electrode 35 is electrically connectedby means of conductor 34 to the anode 24 in the accelerating tube 2, andthe upper hollow electrode 32 is electrically connected to the cathode23 by means of the aforementioned column 23A. Also shown in FIG. 1 is aninduction plate supply 6, which is connected to an induction plate 39located adjacent the lower pulley 38.

Fundamentally, neutrons are produced by means of one or more of severalpossible nuclear reactions within the accelerator tube 2, depending uponthe constituency of the atmosphere 21 and the content of the titaniumtarget 22. For example, if the atmosphere 21 is composed of puredeuterium, and if the target 22 contains only deuterium, then if theatmospheric deuterium is ionized as hereinafter explained, and if thesedeuterium ions are accelerated into the deuterium-impregnated target 22,then the resulting D-D reaction will produce neutrons of approximately2.2 mev. energy. However, if the target is impregnated withsubstantially only tritium, and if the atmosphere 21 contains onlydeuterium, then the wellknown D-T reaction will occur to produce 14.4mev. neutrons. Since the D-T reaction is capable of producing a muchgreater number of neutrons, and since 14.4 neutrons are much moredesirable for well logging purposes, it is the D-T reaction which isgenerally employed for well logging purposes. Of course, the atmosphere21 may be constituted of tritium, and the target 22 may be impregnatedwith deuterium. In such a case, the tritium ions will be acceleratedinto the target deuterium to also produce 14.4 mev. neutrons by means ofthe D-T reaction.

iowever, the heavier tritium atoms require much more energy to beaccelerated into the target 22, and therefore this arrangement is seldomemployed in well logging equipment since power is always at a premiumWhere it must be transmitted thousands of feet downhole to the Van deGraaff generator. One exception to this is the fact that acceleratortubes are sometimes used which employ relatively equal mixtures ofdeuterium and tritium in both the atmosphere 21 and the target 22.

In operation, the induction plate supply 6 functions to apply a highnegative voltage to the induction plate 39 which, in turn, induces acorresponding high positive charge on the lower pulley 38. The motor 5operates to turn the lower pulley 38 in a manner so that the conductivesegments 37A on the belt 37 carry the positiv charge from the lowerpulley 38 to the upper pulley 36. From the upper pulley 36, thispositive charge flows through the lower hollow electrode 35, and theconductor 34, to the anode 24. Thus, as the Van de Graaif generator 4continues to operate, the magnitude of this positive charge on the anode24 increases with respect to both the cathode 23 and the target 22 untilelectrons begin to flow from the cathode 23 to the anode 24. Thiselectron flow across the ionization gap 25 serves to ionize thedeuterium or tritium) in this region of the atmosphere 21, and thusthese positively charged ions are correspondingly attracted towards thecathode 23. However, since the cathode 23 is formed in the manner of amesh, most of the ions pass through the cathode 23 and are acceleratedat very high speeds across the accelerating gap 26, and past thesuppressor rings 12, into the target 22. As hereinbefore explained, itis this acceleration of the hydrogen isotope ions, into the isotopenuclei in the target 22, which produces the neutrons.

It may be seen that a voltage from the induction plate supply 6 is alsoconnected through a suitable corona regulator 7 to apply a negativecharge on the suppressor rings 12. Since secondary electrons will beemitted by the target 22 during its bombardment by the atmospheric ions,it is desirable to suppress this so-called secondary electron emissionto prevent waste of power in the system. However, it is often dsirableto control the output of the induction plate supply 39 to stabilize theneutron output of the accelerator tube 2. Thus, the corona regulator 7acts to apply a constant voltage to the suppressor rings 12,irrespective of variations in the output of the induction plate supply6.

In addition to the so-called beam current (ion flow) across theaccelerating gap 26, the electrostatic generator 4 is preferably adaptedto develop a corona current flow between the upper hollow electrode 32and generator tank 30. Since the upper hollow electrode 32 is not onlyelectrically isolated from the lower hollow electrode 35, but also fromground, the potential on the upper hollow electrode 32 rises withrespect to ground, as the ionization current between the anode 24 andthe cathode 23 ionizes the atmosphere 21, until a current flow developssomewhere between the upper hollow electrode 32 and either the jacket orthe tank 30. Thus, in order to stabilize the beam current, a coronapoint 31 (which is a sharp pointed electrode) is preferably fixed to theinside surface of the tank 30 opposite the upper hollow electrode 32.Since the space between the tip of the corona point 31 and the nearestsurface of the upper hollow electrode 32 is narrower than the spacebetween the upper hollow electrode 32 and any other grounded part of theelectrostatic generator 4, all leakage flow between the upper hollowelectrode 32 and ground (except for the beam current) will beconcentrated between the corona point 31 and the nearest surface of theupper hollow electrode 32. As is well known, it is an inherentcharacteristic of a corona discharge that the magnitude of the currentflow is negligible until the voltage is brought to a certain mag'nitudeVc.

However, as the voltage rises above Vc, the current flow" becomesincreasingly large and therefore, if the voltage established in thesystem between the upper hollow electrode 32 and the corona point 31 issubstantially greater than Vc, relatively large fluctuations in coronacurrent flow will produce only relatively small fluctuations in thevoltage between the corona point 31 and the upper hollow electrode 32.The corona voltage at this point in the system is always equal to thevoltage across accelerating gap 26, since the corona point 31 and thetarget 22 are both substantially at ground potential, the voltage acrossthe accelerating gap .26'will become relatively stabilized by the coronadischarge notwithstanding substantial fluctuations in other parametersof the system.

Referring now to FIGURE 2 wherein is shown a simpler representation ofthe neutron source depicted in FIGURE 1, there may be seen a preferredform of the,

present invention wherein a small self-quenching Geiger- Muller counter49 has been interconnected along conductor 34. As shown, the centralelectrode 41 of the Geiger-Muller counter tube is connected to the lowerhollow electrode 35, and the jacket 43 of the tube 40 is electricallyconnected to the anode 24 in the accelerating tube 2. During operationof the system, positive charge is carried by the belt 37 to the upperpulley 36, and thence as hereinbefore described to the lower hollowelectrode 35. However, no current will flow to the anode 24 due to theinterposition of the counter tube 40. Nevertheless, as the belt 37carries an increasing magnitude of charge to the central electrode 41 ofthe tube 40, potentials are developed within the system across thevarious interelectrode capacitances such as between the centralelectrode 41 and the jacket 43 of the counter tube 46, between the anode24 and the upper hollow electrode 32, between the two hollow electrodes32 and 35, between the column 23A and the anode 24, and between thecathode 23 and the suppressor rings 12 and the target 22. If the counter4-9 is arranged in the system so as to be subject to incident radiation,then each time incident radiation triggers a discharge or pulse withinthe tube, current will flow from the central electrode 41 to the jacket43 of the chamber 40, and thence to the anode 24, and each such discretecurrent flow will produce a resulting discrete flow or pulse ofionization current. correspondingly, each pulse of ionization currentwill produce a discrete flow of beam current which, in turn, willproduce a correspondingly discrete flow, or pulse, of neutrons. It isapparent that the neutron pulses produced in this manner will arise onlywhen the counter tube 40 is struck and triggered by incident radiation.Such radiation may be from a source of alpha, beta or gamma radiationplaced near the counter; or it may be beta radiation from the target orX-rays produced within the neutron source. Or it may be composed ofalpha particles produced in correlation with the neutrons, or gamma raysproduced in the tube 2 structure b capture or inelastic scatteringtherein of the neutrons. Thus, the neutron pulses produced in thismanner will occur at a random rate and in a relatively uncontrolledmanner, although this may be quite unobjectionable for many applicationsof the present invention.

Referring again to FIGURE 2, there is shown therein a lead shield 42which may be disposed about the counter tube in a manner such as toblock substantially all radiation. In such an arrangement, theelectrostatic generator 4 will increase the charge on the centralelectrode 41 of the ionization chamber 40 until the potential across thechamber 41} reaches a certain threshold. When this voltage threshold isattained, the counter tube 40 will tire and a discrete flow of currentwill pass to the anode 24 as hereinbefore described. The current flowthrough the tube 40 wil be relatively short-lived, however, because asthe tube voltage drops due to the flow of current through the tube, thedischarge is quenched or extinguished in the well known manner and willnot again fire until the tube has recovered and the threshold voltage isagain attained. Thus, by choosing a tube or ionization chamber 40 withthe proper threshold characteristics, and by controlling the operationof the electrostatic generator 4, the pulsation rate of the neutronsource can be effectively controlled within reasonable limits.

In another modification of the present invention, a socalled trigger 44composed of a minute quantity of radioactive material may be disposedwithin a recess in the rim of the upper pulley 36, and the shield 42 maybe formed to expose the counter tube 40 in the direction of the upperpulley 36. In such an arrangement, the upper pulley 36 is preferablyfashioned of an opaque material so the recess will function to collimatethe radiation emitted by the trigger 44. Thus, the trigger 44 will sweepa beam of radiation across the counter tube 40 with each revolution ofthe upper pulley 36, and the rotation rate of the upper pulley 36, andthus the pulsation rate of the neutron source, may be selected byadjusting the speed of the motor as desired.

In another alternative of the present invention depicted in FIGURE 2,the conventional Geiger-Muller counter tube maybe replaced by either alight-sensitive or an ultra-violet ray sensitive Geiger-Muller counteror photodiode. In such a case, the function of the trigger 44 may beperformed by a device providing a pulsed light or ultraviolet ray beamat ground potential and may be located at any convenient point outsidethe high voltage terminal rather than within the upper pulley 36 ashereinbefore described. With this embodiment of the invention, the tubemay be of the type which is triggered at the be ginning of the incidentpulse of visible or ultraviolet light; or it may be of the type whichconducts uniformly while the light beam strikes it, and ceases toconduct when the beam is directed elsewhere.

Referring now to FIGURE 3, there may be seen another abbreviatedrepresentation of the neutron source depicted in FIGURE 1, wherein acommercial gas-filled trigger tube 51 is connected to supply thethreshold function. Specifically, the anode and control grid of thetrigger tube 51 are left floating, and the cathode 55 and keep aliveelectrode 54 are, respectively, connected to the anode 24 of theaccelerator tube 2 and the upper hollow electrode 35. When arranged inthis manner, a typical trigger tube 51, such as the CBS 7230 or the EII28, will fire at a reproducible threshold potential of about 200 to 300volts, and will extinguish itself whenever the current developed by theelectrostatic generator 4 is less than a critical value within a rangeof approximately 40 to 100 microamperes. When connected as shown, with amicro-microfarad shunt capacitor 56 between the cathode 55 and thekeepalive electrode 54, and with a 100 micro-microfarad capacitor 57between the keep-alive electrode 54 and the upper hollow electrode 32,this arrangement has been found to produce ionization current pulsesacross the ionization gap 25 at a rate of 500 to 5000 pulses per second,and of a duration of the order of 100 microseconds or less per pulse,depending upon the particular component values selected for the circuit.The values herein given for capacitors 56 and 57 may be varied asdesired.

FIGURE 4 shows a variation of the neutron source depicted in FIGURES 13,wherein the upper hollow electrode 32A is similar to the previouslydepicted upper hollow electrode 32, but is not electrically connected tothe cathode 23 of the accelerator 2 as hereinbefore described. Instead,a trigger tube 61 is arranged with its keep-alive electrode 64electrically connected to the cathode 23 of the accelerator tube 2, andwith the cathode 65 of the trigger tube 61 connected electrically to themodified upper hollow electrode 32A. A shunt capacitor 65, of suitablevalue, is also connected across the trigger tube 61, and the controlgrid 62 and anode 63 of the trigger tube 61 are also left floating asdepicted in FIGURE 3.

The various forms of the present invention, which are embodied in thecircuits depicted in FIGURES 24, will all function adequately for manypurposes. However, unless the various components forming the depictedcircuits are selected for specific characteristics, the neutron outputof the accelerator tube 2 may extinguish relatively slowly after eachoutput pulse. Furthermore, the accelerator tube 2 may begin to produceneutrons, in limited quantities, prior to each firing of whatevertriggering device is utilized (Geiger-Muller tube 4%, trigger tube 51,or trigger tube 61). In such a case, the neutron pulses emitted by theaccelerator tube 2 may not be as sharp as may be desired. Nevertheless,a neutron source incorporating any of these circuits will operate toproduce clearly defined neutron pulses at a very high frequency.

FIGURE 5 shows another embodiment of the present invention wherein atrigger tube 71, of the type herein depicted in FIGURES 3-4, is arrangedwith its cathode connected to the anode 24 of the accelerator tube 2,

and with its plate or anode 73 connected via conductor 34 to the lowerhollow electrode 35 and the upper pulley 35 of the electrostaticgenerator 4. Both the keep-alive electrode 74 and the control grid 72,of the trigger tube 71, are connected to the cathode 78A of a coronaregulator tube 78, which has its plate 78B connected to the anode 73 ofthe trigger tube 71. The cathode 78A, of the corona regulator tube 78,is also connected to one side of a shunt capacitor 79A, the other sideof which is connected to the cathode '75 of the trigger tube 71. In thisarrangement, the corona regulator tube 78 is chosen so that itsthreshold voltage is lower than the threshold voltage of the triggertube 71. As the electrostatic generator 4 operates to deliver anincreasing charge to the anode 73 of the trigger tube 71, the potentialincreases across the corona regulator tube 78 at a rate determined bythe size of the interelectrode capacitance between the upper hollowelectrode 32 and the cathode 23 of the accelerator tube 2. Thus, acapacitor 79B is preferably provided herein to establish this rate atthe value desired, since the rate will determine the pulse frequency ofthe neutron source.

When the threshold voltage across the corona regulator tube 78 isattained, the corona regulator tube 78 will develop a current flow fromthe anode 73 of the trigger tube '71 to its control grid 72 and itskeep-alive electrode 74. It is an inherent characteristic of any coronaregulator tube, however, that its conducting potential is substantiallylower than its threshold potential, and thus the resulting voltage dropacross the corona regulator tube 78 will correspondingly produce asudden increase in the potential across the shunt capacitor 72A. Thisincrease in potential is also produced between the cathode 75 andkeep-alive electrode 74, of the trigger tube 71, to effectively triggera short burst, or pulse, of current flow between the anode 73 and thecathode 75 of the trigger tube 71. However, any current flow between theanode 73 and the control grid 72, of the trigger tube 71, effectivelyshorts out the corona regulator tube 78 and extinguishes it by reducingthe potential between its cathode 78A and plate 788 to a levelsubstantially below its conducting potential. This eflfectively quenchesthe trigger tube 71, and cuts off current flow between its anode 73 andcontrol grid 72, and therefore interrupts current flow to the anode 24of the accelerator tube 2. As hereinbefore explained for capacitor 57 inFIGURE 3, capacitor 79B acts to determine (according to itscharacteristics) the frequency at which the overall circuit depicted inFIGURE 5 will deliver pulses of current to the anode 24 of theaccelerator tube 2.

Referring now to FIGURE 5, there may be seen a preferred embodiment ofthe present invention wherein a neutron source composed of anaccelerator tube 2 and an electrostatic generator 4 is representedsubstantially as depicted in FIGURE 1. As hereinbefore described, theinduction plate supply 6 induces a positive charge on the belt 37 as itpasses the induction plate 39, and this positive charge is carried tothe anode 24 of the accelerator tube 2 by means of the upper pulley 36,the lower hollow electrode 35, and the conductor 34. After a suflicientamount of charge has been delivered in this manner to produce thetriggering action hereinbefore described, the accumulated charge isdelivered to the anode 24-, of the accelerator tube 2. The difference inpotential across the ionization gap 25 then is caused to exceed athreshold magnitude, whereupon electrons will flow from the cathode 23,of the accelerator tube 2, to its anode 24 by means of cold cathodeemission. It is apparent that due to the flow of current to the anode 24thereby charging the interclectrode capacitances, a potential differencewill not only be developed across the ionization gap 25, but alsobetween other electrodes in the system such as the acceleration gap 26,between the suppressor rings 12 and the target 22, and between thecathode 23 and the suppressor rings 12. When current flows across theionization gap 25, the difference in potential between the cathode 23and the anode 24 is reduced in proportion to the amount of chargetransferred. Thus, when the trigger means ceases to conduct, thepotential difference between cathode 23 and anode 24 drops immediatelyto the threshold where conduction ceases. Thus, ionization in theacceleration tube ceases and neutron productions stops until the cycleis repeated after charge has again accumulated to once more fire thetrigger means. However, this potential difference is by no meanseliminated.

In another embodiment of the present invention as depicted in FIGURE 6,there may be seen a pulse generator 100 which is connected to a couplingcondenser 102, which is preferably large in capacitance relative to theinterelectrode capacitance, said interelectrode capacitance beinghereinafter referred to as C Capacitance C exists between the suppressorrings 12 and the cathode 23. The coupling condenser 102 is, in turn,coupled between the load resistor 104 usually connected in thesuppressor ring circuit and the suppressor rings 12. The pulse generator100 may be adapted to apply a sequence of negative pulses to thecoupling condenser 102, at either a fixed preselected frequency, or at arate determined by surface located control apparatus not shown in thedrawings.

These negative pulses on the suppressor rings 12 produce negative pulseson the cathode 23 due to the capacitance coupling provided byinterelectrode capacitance C Since positive current is delivered to theanode 24 by the electrostatic generator 4, the cathode 23 is alwaysnegatively charged to some extent with respect to the charge on theanode 24, except of course when the entire system is inactivated.However, each time the pulse generator .100 applies a pulse ashereinbefore described, the cathode 23 will become negatively chargedrelative to its normal condition so that its potential relative to theanode 24 exceeds the normal threshold voltage of the ionization gap 25.This produces an almost instantaneous spurt, or pulse, of electrons andin current between the cathode 23 and the anode 24; which in turnprovides a corresponding pulse of deuterium ions towards the target 22.If the applied pulse is large enough, all of the ion current which wouldnormally flow between cathode 23 and anode 24 during the intervaldelineated by two such pulses produced and applied to the system by thepulse generator 100, flows instead during the interval of each of thesenegative pulses. The magnitude of the required pulse obviously dependsupon the Van de Graalf'current, the desired repetition rate, and pulseduration. It has been found that an applied pulse of 3000 volts willproduce an ion current pulse which will in 20 microseconds substantiallydischarge the interelectrode capacitance C subsisting between anode 24and cathode 23. After a pulse of this type has occurred, no ionizationcurrent will appear across the ionization gap 25 until another negativepulse is supplied by the pulse generator 100, or until the electrostaticgenerator 4 succeeds in recharging C up to its threshold potential. Witha generator charging current of 30 microamperes or less it has beenfound that this does not occur for about 1000 microseconds. This secondalternative can be eliminated by causing the pulse generator 100 todeliver its negative pulses at such a frequency, or at such an amplitude(or both), that C is never recharged to its threshold potential withinthe time interval subsisting between pulses. Thus, neutrons will beproduced, by this embodiment of the present invention, only in the formof sharply defined pulses, and only in response to pulses applied by thepulse generator 100 to the coupling condenser 102. The interelectrodecapacitance (hereinafter referred to as C between the lower hollowelectrode 35 and the tank 30 of the electrostatic generator 4, tends tohold the potential of the anode 24 constant during the occurrence of thepulse supplied by the pulse generator-100. This is an important featureof the embodiment of the present invention depicted in a 10 FIGURE 6,since the Van de Graatf electrostatic generator 4 is a constant-currentpower supply, and since the neutron source depicted includes provision(such as the corona point 31 hereinbefore discussed) for stabilizingcertain voltages such as the voltage across the accelerating gap 26.

It will be apparent to those skilled in the art that the variousinterelectrode capacitances in the neutron source, by operation of theelectrostatic generator 4, are all relative to each other in magnitude,and may vary in magnitude according to variations in any one or more ofsuch capacitances. Thus, alternative forms of the present inventiondepicted in FIGURE 6 may be provided by applying triggering pulses toone or more of these interelectrode capacitances, other than C ashereinbefore described. For example, FIGURE 7 shows a variation of thecircuitry and system depicted in FIGURE 6, wherein an auxiliaryelectrode 110 is mounted within the electrostatic generator 4, oppositethe lower hollow electrode 35, and near the tank 30 of the electrostaticgenerator 4. This auxiliary electrode 110 is preferably insulated fromthe tank 30 (which is at ground potential) by an insulator 12, and isconnected to a pulse generator 114 which, in turn, is adapted togenerate positive pulses analogous to the manner previously describedfor the pulse generator shown in FIGURE 6. In FIGURE 7, the positivepulse generator 114 is shown located within the electrostatic generator4, but this is not a significant feature of the present invention.

During operation of the electrostatic generator 4, the current build-upon the anode 24, of the accelerator tube 2, will develop charges on thevarious interelectrode capacitances throughout the system ashereinbefore explained. If the pulse generator 114 is then caused toapply a sharp positive pulse of suitable magnitude to the auxiliaryelectrode 110, as depicted in FIGURE 7, no change will occurinstantaneously in the charge on C Since the charge on any capacitor isalways the product of the capacitance of such capacitor and the voltageacross it, and since the capacitance of C is constant, it follows thatthe voltage across 0, tends to remain constant during the applied pulse.Since the voltage across C is unchanged, then the anode 24 will bedriven further positive, with respect to the cathode 23 (and the target22 and suppressor rings 12, etc.) to the extent of the positive pulsesupplied by the pulse generator 114. Given proper operating conditions(such as the proper rate of accumulation of charge on the anode 24), thepotential difference across C will be caused to responsively rise abovethe threshold for ionization current flow across the ionization gap 25,and produce a burst of ions as hereinbefore described. Furthermore,given a suitable pulse frequency, all of the current which would, in theabsence of pulses, be caused to flow across the ionization gap 25,during the interval between two pulses from the pulse generator 114,will be caused to flow during the triggering pulse. Since C is, ineffect, completely discharged by the triggering pulse provided by thepulse generator 114, no ionization current flows until the next pulseoccurs, with the result that all neutrons are produced in bursts ofpulses synchronous with the voltage pulses applied to electrode 110.

Referring again to FIGURE 7, it may be seen that the pulse generator 114may be adapted to supply al o a negative trigger pulse to the couplingcondenser 102, of the suppressor ring 12 circuit, in the manner shown inFIG- URE 6. This negative pulse should, of course, be appliedsimultaneously with the application of the positive pulse to theauxiliary electrode 110. However, the negative pulse need notnecessarily be supplied by the pulse generator 114, but may be developedby a separate pulse source.

FIGURE 8 shows another alternative form of the present invention,wherein a positive trigger pulse is coupled to the anode 24 by means oftransformer coupling. This transformer coupling may be effected by meansof a core 120 which is preferably formed of a substance which may bemagnetized but which is substantially nonconductive, though underfavorable circumstances, adequate coupling may be effected without amagnetic core. The core 120, which may form the support column for theupper and lower pulleys 36 and 33, also supports a primary winding 122which is connected to receive trigger pulses from any suitable pulsegenerator 124, and a secondary winding 126 which is interconnectedbetween the lower holow electrode 35B and the conductor 34A connected todeliver current to the anode 24 of the accelerator tube 2. The pulsesapplied to the primary winding 122 may be of either polarity, providedthat the primary and secondary windings 1.22 and 126 are wound, relativeto each other, so that the secondary winding 126 delivers pulses ofpositive polarity. The lower hollow electrode 358 depicted in FIGURE 8functions in a manner similar in all essential aspects to the lowerhollow electrode 35 depicted in FIGURE 1. However, it may be modified inform to accommodate the core 120. Similarly, the upper hollow electrode32B may be modified in form to accommodate the core 120. The apparatusdepicted generally in F1"- URE 8 functions in a manner similar to theapparatus depicted in FIGURE 5. That is, the positive pulses deliveredby the secondary winding 126 act to raise the potential differencebetween the anode 24 and the cathode 23 during the pulse and therebytrigger the flow of current which discharges the interelectrodecapacitances where charge has accumulated due to the operation of theelectrostatic generator 4 in the same manner that the positive pulsesapplied to the auxiliary electrode shown in FIGURE 7 operated ashereinbefore described.

In particular, the present invention is not limited in its applicationto sources employing a Van de Graafi' or other electrostatic generatoras a source of ion accelerating voltage. The principles of the presentinvention may equally well be applied to operate a given accelerationtube with any conventional high voltage generator, provided that agenerator is selected which has a high output impedance. It is importantthat excessive currents are not caused to flow when the neutron sourceis triggered. To avoid this, it is not only necessary that a highresistance be connected in series with the output of the high voltagegenerator in order that its operation be analogous, in every respect, tothe operation of the Van de Graatt generator as herein described.

Moreover, the present invention may be applied to the pulsed operationof acceleration tubes differing in design from the preferred embodimentherein disclosed. As will be understood by those skilled in the art, thepresent invention may be employed in exactly the same way with anyneutron source which is operated in such a way that a single voltagesupply provides both the ion producing and ion accelerating functionafter the manner disclosed in the copending application for LettersPatent filed by A. H. Youmans and Eric C. Hopkinson, on Aug. 26, 1959,and bearing Ser. No. 836,098, and which issued as US Patent No.3,117,314.

Numerous other variations and modifications may obviously be madewithout departing from the present invention. Accordingly, it should beclearly understood that those forms of the invention described above andshown in the figures of the accompanying drawings are illustrative only,and are not intended to limit the scope of the invention.

What is claimed is:

1. A source of neutrons comprising an ion beam accelerator and aconstant current generator;

said accelerator including a static atmosphere substantially composed ofa heavy isotope of hydrogen,

ionization means connected to said generator to ionize said atmosphere,

12 and a target containing a heavy isotope of hydrogen arranged toreceive atmosphere ions;

said source further including pulsing means interconnected with saidionization means and said generator to intermittently energize saidionization means in a manner such that ionization means ionizes saidatmosphere in a functionally related intermittent manner.

2. A source of neutrons comprising an ion beam accelerator and aconstant current electrostatic generator;

said accelerator including a static atmosphere substantially composed ofa heavy hydrogen isotope,

means including an anode and a cathode arranged to produce an ionizingflow of electrons in said atmosphere,

and a target containing a heavy hydrogen isotope arranged to receivehydrogen isotope ions produced in said atmosphere by said electron flow;

said generator including and endless rotatable belt,

means for developing an electric charge on said belt at a firstlocation,

and a collector electrode for receiving such electric charge from saidbelt at a second location;

said neutron source also comprising pulsing means interconnected withsaid anode and said collector electrode to intermittently conduct saidelectric charge therebetween to produce said ionizing flow of electronsin a functionally related intermittent manner.

3. The apparatus in claim 2 wherein said pulsing means includes a chargeconducting means having a predetermined threshold.

4. The apparatus in claim 2 wherein said pulsing means includesconducting means having a predetermined threshold,

and

quenching means arranged and adapted to respond to conduction of saidelectric charge by said conducting means in a manner to interrupt suchconduction after a predetermined time interval.

5. The apparatus in claim 3 wherein said charge con ducting meansincludes a Geiger-Muller counter having a central electrode connected tosaid collector electrode in said generator and having a jacket electrodeconnected to said anode in said accelerator.

, 6. The apparatus in claim 4 wherein said Geiger-Muller counter isshielded from incident radiation.

7. The apparatus in claim 3 wherein said pulsing means includes a firstcapacitance connected between said collector electrode and saidaccelerator cathode, and

a second capacitance connected across said charge conducting meansbetween said collector electrode and said accelerator anode.

8. The apparatus in claim 7 wherein said first capacitance is greaterthan said second capacitance.

9. The apparatus in claim 5 wherein said Geiger-Muller counter isdisposed in a radiation shield having an aperture adjacent said endlessbelt at said second location, and

including actuating means operated by said belt and adapted toperiodically cause said Geiger-Muller counter to conduct saidelectric'charge.

10. A source of neutrons comprising an ion beam accelerator and aconstant current electrostatic generator;

said accelerator including a static atmosphere composed of a heavyhydrogen isotope,

means including an anode and a cathode arranged to provide an ionizingflow of electrons in said atmosphere,

a target containing a heavy hydrogen isotope arranged to receivehydrogen isotope ions produced in said atmosphere by said electron flow,and an intermediate electrode capacitively arranged relative to saidcathode;

said generator including an endless rotatable belt,

means for developing an electric charge on said belt at a firstlocation,

a collector electrode arranged and adapted to receive such electriccharge from said belt at a second location;

and pulsing means interconnected with said collector electrode in saidaccelerator for sequentially allowing and interrupting said ionizingflow of electrons.

11. The apparatus in claim wherein said pulsing means includes anelectric pulse generator arranged and adapted to sequentially apply tosaid collector electrode electric pulses of an amplitude and polaritysuch as to cause said electron flow to occur substantially only duringeach of said pulses.

12. A source of neutrons comprising an ion beam accelerator and aconstant current electrostatic generator;

said accelerator including a static atmosphere composed of a heavyhydrogen isotope,

means including an anode and a cathode arranged to provide an ionizingflow of electrons in said atmosphere,

and a target containing a heavy hydrogen isotope arranged to receivehydrogen isotope ions produced in said atmosphere by said electron flow;said generator including an endless rotatable belt,

means for developing an electric charge on said belt at a firstlocation,

' a first electrode arranged and adapted to receive such electric chargefrom said belt at a second location,

and a second electrode capacitively arranged in said generator relativeto said first electrode;

and pulsing means interconnected with said second elect-rode in saidgenerator for sequentially allowing and interrupting said ionizing flowof electrons.

13. The apparatus in claim 12 wherein said pulsing means includes anelectric pulse generator arranged and adapted to sequentially apply tosaid second electrode electric pulses of an amplitude and polarity suchas to cause said electron flow to occur substantially only during eachof said pulses 14. The apparatus in claim 13 wherein said acceleratoralso comprises an electrode arranged intermediately of said target andsaid means including an anode and a cathode, and wherein said electricpulse generator is also interconnected with said electrode in saidaccelerator.

15. The apparatus in claim 14 wherein said electric pulse generator isfurther adapted to apply pulses of a first polarity to said secondelectrode in said generator and coincidently therewith to apply pulsesof a second polarity to said intermediately arranged electrode in saidaccelerator.

16. A source of neutrons comprising an ion beam accelerator and aconstant current electrostatic generator;

said accelerator including a static atmosphere composed of a heavyhydrogen isotope,

means including an anode and a cathode arranged to provide an ionizingflow of electrons in said atmosphere,

and a target containing a heavy hydrogen isotope arranged to receivehydrogen isotope ions produced in said atmosphere by said electron flow;said generator including an endless belt,

means for rotating said belt,

means for developing an electric charge on said belt at a firstlocation,

and an electrode for receiving such charge from said belt at a secondlocation;

said neutron source also comprising pulsing means including a couplingtransformer having one of two windings interconnected between saidelectrode in said generator and said anode in said accelerator, and anelectric pulse generator connected to the other of said two windings ofsaid transformer.

17. A source of neutrons comprising an ion beam accelerator and aconstant current electrostatic generator;

said accelerator including a static atmosphere composed substantially ofa heavy isotope of hydrogen, ionization means arranged to provide anionizing flow of electrons in said atmosphere,

and a target containing a heavy isotope of hydrogen arranged to receiveatmosphere ions;

said source further including pulsing means comprising an electrodecapacitively arranged relative to said ionization means, and

an electric pulse generator connected to sequentially apply electricvoltage pulses to said elec trode to cause said flow of electrons tooccur substantially only during each of such pulses.

18. The apparatus in claim 17 wherein said ionization means includes ananode and a cathode, and wherein said electrode is capacitively arrangedrelative to said anode.

19. The apparatus in claim 17 wherein said ionization means includes ananode and a cathode, and wherein said electrode is capacitively arrangedrelative to said cathode.

20. Apparatus for producing pulses of radiation comprising a powersupply continuously operating to produce charge flow at not more than apreselected rate; storage means receiving a charge from said powersupply; and

an ion accelerator including a static atmosphere of hydrogen, ionizationmeans having a voltage threshold and including an anode and a cathode,said anode being interconnected with said storage means, and a targetarranged to receive hydrogen ions; said apparatus further comprisingpulsing means for intermittently increasing the voltage of said anoderelative to said cathode above said threshold to transfer between saidanode and said cathode a discrete amount of charge equal to the chargeflow delivered by said power supply during the interval since thepreceding pulse.

21. Apparatus as described in claim 20, wherein said storage meansincludes a first capacitance connected to the output of said powersupply,

and wherein said pulsing means includes a second capacitance and asource of pulsated voltage, said second capacitance being connected tosaid first capacitance and to said source of pulsated voltage, saidsecond capacitance also being connected in series with said power supplyand said first capacitance.

22. Apparatus as described in claim 21, wherein said source of pulsatedvoltage produces voltage pulses at a predetermined rate.

23. Apparatus for cyclically producing pulses of radiation with apredetermined period, said apparatus comprising a power supplycontinuously operating to produce charge flow at not greater than apreselected maximum rate;

storage means receiving charge from said power supply; and

15 an ion accelerator including a static atmosphere of hydrogen,ionization means having a voltage threshold and including an anode and acathode, said anode being interconnected with said storage means, and atarget arranged to receive hydrogen ions; said apparatus furthercomprising pulsing means for cyclically increasing the voltage of saidanode relative to said cathode above said threshold to transfer betweensaid anode and cathode a discrete amount of charge equal to the chargedelivered by said power supply during said predetermined cycle period.24. A source of neutrons comprising a power supply continuouslygenerating a flow of charge; an ion accelerator including a staticatmosphere substantially composed of a heavy isotope of hydrogen,ionization means connected to to ionize said atmosphere, and a targetcontaining a heavy isotope of hydrogen arranged to receive atmosphereions; and pulsing means interconnected with said power supply and saidionization means to cause said ionization means to produce functionallyrelated discrete pulses of atmosphere ions. 25. A source of radiationcomprising a power supply continuously operating to produce charge flowat not more than a preselected maximum rate; storage meansinterconnected to receive at least a preselected amount of charge flowfrom said power pp y; an ion accelerator including a static atmosphereof hydrogen, ionization means having a voltage threshold and includingan anode and a cathode, said anode being interconnected with saidstorage means and said power supply, and

said power supply 16 a target arranged to receive hydrogen ions; saidsource of radiation further comprising pulsing means for intermittentlyincreasing the voltage of said anode relative to said cathode above saidthreshold to transfer between said anode and said storage means adiscrete amount of charge flow equal to said preselected amount ofcharge in said storage means. 26. A source of radiation comprising apower supply continuously operating to produce charge flow at not morethan a preselected maximum rate; storage means interconnected to receiveat least a preselected amount of charge fiow from said power supplyduring a first predetermined time interval; an ion accelerator includinga static atmosphere of hydrogen, ionization means having a voltagethreshold and including an anode and a cathode, said anode beinginterconnected with said storage means and said power supply, and atarget arranged to receive hydrogen ions; said source of radiationfurther comprising pulsing means for increasing the voltage of saidanode relative to said cathode above said threshold during a secondshorter predetermined time interval to transfer between said anode andsaid storage means during said second interval a discrete amount ofcharge flow equal to said preselected amount of charge in said storagemeans.

References Cited by the Examiner UNITED STATES PATENTS ARCHIE R.BORCHELT, Primary Examiner.

JAMES W. LAWRENCE, RALPH G.

NTLSON,

Examiners.

2. A SOURCE OF NEUTRONS COMPRISING AN ION BEAM ACCELERATOR AND ACONSTANT CURRENT ELECTROSTATIC GENERATOR; SAID ACCELERATOR INCLUDING ASTATIC ATMOSPHERE SUBSTANTIALLY COMPOSED OF A HEAVY HYDROGEN ISOTOPE,MEANS INCLUDING AN ANODE AND A CATHODE ARRANGED TO PRODUCE AN IONIZINGFLOW OF ELECTRONS IN SAID ATMOSPHERE, AND A TARGET CONTAINING A HEAVYHYDROGEN ISOTOPE ARRANGED TO RECEIVE HYDROGEN ISOTOPE IONS PRODUCED INSAID ATMOSPHERE BY SAID ELECTRON FLOW; SAID GENERATOR INCLUDING ANDENDLESS ROTATABLE BELT, MEANS FOR DEVELOPING AN ELECTRIC CHARGE ON SAIDBELT AT A FIRST LOCATION, AND A COLLECTOR ELECTRODE FOR RECEIVING SUCHELECTRIC CHARGE FROM SAID BELT AT A SECOND LOCATION; SAID NEUTRON SOURCEALSO COMPRISING PULSING MEANS INTERCONNECTED WITH SAID ANODE AND SAIDCOLLECTOR ELECTRODE TO INTERMITTENLY CONDUCT SAID ELECTRIC CHARGETHEREBETWEEN TO PRODUCE SAID IONIZING FLOW OF ELECTRONS IN AFUNCTIONALLY RELATED INTERMITTENT MANNER.